This chapter discusses plane wavesPlane waves that propagate in an unbounded medium, starting from Maxwell’s equationsMaxwell’s equations. Applying the initial and boundary conditions that characterize the medium to Maxwell’s equationsMaxwell’s equations gives a solution for the electric and magnetic fieldsMagnetic field of plane wavesPlane waves. The propagation characteristics are examined by dividing the medium into free space, dielectricsDielectrics, and lossy mediaLossy medium including good conductorsConductors. This chapter also examines the mathematical solutions for the reflection and transmission of plane wavesPlane waves for normal andReflectionoblique incidence oblique incidenceOblique incidence depending on the polarizationPolarization. The formation of standing wavesStanding wave at complete reflection provides information about the distribution of electric and magnetic fieldsMagnetic field, and their ratio leading to the input impedanceInput impedance provides a physical basis for the resonant 1/4 wavelengthWavelength absorbers.